An electromagnetic wave caused by a high-frequency component of a digital clock signal is radiated from a digital device such as an LSI into a space and may have an influence on surrounding electronic devices. Especially, in an electronic device in which a digital circuit and an analog circuit (or a wireless circuit) are mounted, electromagnetic noise may be radiated and have an influence on the signal quality of a receiving antenna when an electromagnetic wave that corresponds with a wireless frequency of the electromagnetic wave radiated from the LSI that is the digital circuit flows into the wireless device. Therefore, a technique for reducing the electromagnetic noise radiated from the LSI is important for the wireless device in which the digital circuit and the analog circuit are mounted, to maintain the signal quality.
A power source decoupling technique is known as the technique for reducing the electromagnetic noise radiated from the LSI. An example of the technique is shown in FIG. 7.
In FIG. 7, a power source decoupling structure 100 is a structure for reducing electromagnetic noise radiated from a power source terminal 103 of an LSI 102 implemented on a substrate 101. The power source decoupling structure 100 is configured such that a capacitor 104 for decoupling is connected to the power source terminal 103 of the LSI 102 and a ground terminal 105. The ground terminal 105 provided on the surface of the substrate 101 is electrically connected to a ground layer 107 provided in the substrate 101, through a via hole 106 extending in the thickness direction of the substrate 101.
Because influential electromagnetic noise is usually high-frequency wave, the high-frequency electromagnetic noise radiated from a power source system of the LSI 102 flows into the ground terminal 105 provided on the surface of the substrate 101, the via hole 106 formed in the substrate 101, and the ground layer 107 in order by using the capacitor 104. As a result, it is possible to prevent the noise from leaking to other areas.
By the way, an electromagnetic shield technique is also used as the technique for reducing the electromagnetic noise radiated from the LSI. The electromagnetic shield technique is basically such that a metal cap covers the whole LSI and is connected to the ground of the substrate. An example thereof is shown in FIG. 6. In FIG. 6, the same symbols shown in FIG. 7 denote the parts corresponding to the elements of FIG. 7.
FIG. 6A shows an example of a typical metal shield structure 200. The metal shield structure 200 is provided on the substrate 101 and includes a metal shield 201. The shield 201, as shown in FIG. 6B, is configured by a top panel 202 that is larger than the LSI 102 implemented on the substrate 101 and four side panels 203 that are respectively provided along four sides of the top panel 202 and cover the four side surfaces of the LSI 102. The substrate 101, though not shown, has a multilayer structure, and as shown in FIG. 6C, the ground layer 107 is formed at an internal layer of the substrate 101. A plurality of via holes 204 extending in the thickness direction of the substrate 101 are formed in the substrate 101. These via holes 204 are arranged in a wall shape and at a narrow pitch along the side panels 203 of the shield 201. The shield 201 is electrically connected (grounded) to the ground layer 107 through the via holes 204.
Therefore, the LSI 102, as shown in FIG. 6C, is sandwiched between the top panel 202 of the shield 201 and the ground layer 107 formed in the substrate 101 at the top surface and the under surface of the LSI 102. The side surfaces of the LSI 102 are surrounded by the side panels 203 of the shield 201 and the wall of the via holes 204 (the via holes 204 arranged in a wall shape). Thus, the perimeter of the LSI 102 is surrounded by the conductor plates (metal plates) in the metal shield structure 200. Therefore, the electromagnetic noise radiated from the LSI 102 is shielded. As a result, the amount of noise radiated circumferentially by the LSI 102 decreases. That is, it is possible to reduce the electromagnetic noise radiated from the LSI 102.
However, in the metal shield structure 200, the LSI 102 is covered by the metal cap. Therefore, there is a drawback that the radiation efficiency of heat generated by the LSI 102 decreases. For this reason, a shield structure that is configured such that a pair of side panels 203 arranged at two facing side surfaces of the shield 201 is removed to release heat therefrom is proposed in Japanese Patent No. 3738755. This shield structure is supposed to be effective for decreasing the Electro-Magnetic Interference (EMI) generated by the LSI itself.    [Patent Document 1] Japanese Patent No. 3738755